The Bay Area Rapid Transit District, known as BART, adopted a policy yesterday that bars officials from interrupting cell service in subway stations except in "extraordinary circumstances," such as when there is evidence of imminent unlawful activity that threatens the safety of people, property destruction, or disruption of subway service.

"Today BART took an important step in responding to legitimate concerns raised by its August 11, 2011, interruption of wireless service. As the policy BART adopted recognizes, communications networks that are open and available are critical to our democracy and economy," FCC Chairman Julius Genachowski wrote in a statement.

The FCC must ensure that public safety officers have access to communications networks in emergencies, he said. "For interruption of communications service to be permissible or advisable, it must clear a high substantive and procedural bar."

"The legal and policy issues raised by the type of wireless service interruption at issue here are significant and complex," he added. "I have asked Commission staff to review these critical issues and consider the constraints that the Communications Act, First Amendment, and other laws and policies place upon potential service interruptions. We will soon announce an open, public process to provide guidance on these issues."

BART police shut the cell service in four San Francisco stations in mid-August to thwart a protest over the fatal shooting of Charles Blair Hill, who BART police said went after them with a knife before an officer shot him on July 3. BART police also had fatally shot Oscar Grant in January 2009 as he was being restrained.